Maddy Morrissey, a Website designer in Herndon, Virginia, can laugh now when she looks back on her ambitious maternity-leave plans. She had every intention of chronicling each day on film, making her own birth announcements, and repainting the nursery a lovely pistachio green. As it turned out, the film wasn't developed for a year, her daughter Erin's bedroom stayed its original blue, and the beautiful birth announcements are still sitting on a shelf.

What got in her way? Oh, just a few unplanned events, including a C-section that limited her mobility for weeks; a painful little toe, broken when she hit her foot against the wall on the way to a middle-of-the-night nursing session; and several trips back to the hospital to treat Erin's severe jaundice.

"My coworkers were jealous of my maternity leave," says Morrissey. "They kept saying, 'You're so lucky to have three months off,' as if it were some sort of vacation in the Bahamas! It was the complete opposite."

Like most pregnant women, you're undoubtedly looking forward to your much-deserved hiatus from work — but be prepared for surprises that can sidetrack your best intentions. What's more, you'll probably discover that the weeks fly by in a whirlwind: Just as you're starting to feel settled and are really enjoying your baby, it will be time to go back to work.

When my daughter, Zoe, was born two-and-a-half years ago, I had all sorts of fantasies about what she and I would do during the ten weeks I was taking off from my newspaper job. The reality? If I managed to bathe both my daughter and myself on the same day, it was a major accomplishment.

But now, thinking about what I didn't get done, I have no regrets. And neither should you, if you don't quite get around to brisk walks through the park, Oprah's latest book-club selection, or spring-cleaning your house from top to bottom.

Here are five good reasons you can't do it all.

Not enough time

When you have a baby, your 9-to-5 job is replaced by a round-the-clock gig. Babies need a lot — a lot of feeding, a lot of burping, a lot of diapering, a lot of love, a lot of you. In short, a lot of time. Like many newborns, Zoe demanded to be breast-fed every two to three hours and would then nurse for an hour at a time. Unfortunately, she stuck to that schedule day and night for the entire ten weeks of my leave — so I had only the briefest slivers of time for anything else.

Not enough sleep

It can take a while for babies to learn that they should snooze when it's dark. Until your child is regularly sleeping through the night, which can take several months, expect to be exhausted.

"Women are trying to get too much done on a mere four to five hours of interrupted sleep," says Brenda Hunter, Ph.D., a developmental psychologist and author of The Power of Mother Love. That's hardly enough rest for you to bounce back into prepregnancy shape with daily workouts, make that souffle you've always longed to try, or learn to speak Italian.

A change in priorities

The pleasant parts of caring for a newborn — and the famous nesting instinct that kicks in — are good reasons to ignore your long to-do list. If you'd rather relax and smell the top of the baby's head than do the dishes, who can blame you? Deborah Highland West, a newspaper reporter in Madison, Tennessee, figured she'd have the discipline to write a book during her spare time, but she quickly lost the inclination.

"After Savannah was born, I never even looked at my notes. I wanted to hold her constantly and didn't want to be doing anything else."

You'll be falling in love

"You can't predict what that's going to be like any more than you can predict a romance with an adult," says Nina Barrett, author of I Wish Someone Had Told Me: A Realistic Guide to Early Motherhood. In her research, Dr. Hunter found that most new mothers were completely unprepared for the emotional impact of having a baby — what she calls "the equivalent of a psychic earthquake."

Parenthood is a different way of doing business

Similarly, if you're used to punching a clock, making the mental shift to the anything-goes schedule of caring for an infant can be a challenge. The qualities that are valued in the workplace — control, organization, planning, and productivity — simply aren't realistic once you're at home with a baby.

"Work is intellectual; caring for a child is emotional," says Dr. Hunter. "If you go into your maternity leave with the same achievement-oriented attitude that you had as a working person, you're going to go nuts," adds Barrett. And you can't depend on positive feedback for motivation — there won't be any e-mails acknowledging what a good job you're doing managing soggy diapers.

I admit that, though I missed my baby, I felt a bit relieved to get back to the adult work world, which was less chaotic than life at home. There's no shame in rejoicing when you get to take off the burpy cloth and put panty hose back on. "I yearned to be able to think about ideas in a quiet place and be productive in a different way," admits Brooke Russell, who returned to work as a writer in Chicago after four months at home with Grace, now 1.

Maybe you're one of the rare breed of women who'll careen effortlessly through your postpartum days with a spit-up-free baby who sleeps through the night, a sparkling clean house, and a French manicure. But in case you're not, save room in your maternity-leave plans for the surprises that may pre-occupy you. Give yourself permission to sleep whenever possible, to coo and snuggle, to heal and deal.